TUESDAY, May 23 (HealthDay News) -- For people under the age of 80, a colonoscopy might provide benefits that last beyond the 10 years now recommended as the interval between procedures, a new study finds.

On the other hand, a second study finds the screen may only provide minimal benefit to people aged 80 or older.

While both findings provide valuable guidance to patients and doctors, the decision to undergo a potentially lifesaving colonoscopy "is best determined by the individual and the physician. Anyone who is not being screened should have that conversation with their doctor," said Timothy R. Church, a professor of environmental health sciences at the University of Minnesota School of Public Health in Minneapolis.

Church wrote an editorial that accompanies both studies, which are reported in the May 24/31 issue of the Journal of the American Medical Association.

A colonoscopy looks for colon polyps that could become cancerous over time. One of the studies was done by researchers at the University of Manitoba in Winnipeg, Canada. They found that the benefit of a colonoscopy, in terms of reducing the risk of developing cancer, may last beyond the 10 years now recommended as the interval between procedures.

The second study, done at Virginia Mason Medical Center in Seattle, found that colonoscopy provides minimal benefit in terms of extended life expectancy for people over 80.

Both studies were observational -- that is, they simply looked at the medical records of people who underwent the procedure in clinical practice, instead of following people over time, as might happen in a controlled trial.

The Manitoba study involved data on nearly 35,000 Canadians whose colonoscopies turned up no polyps. Over the next 10 years, the incidence of colon cancer in those individuals was 72 percent lower than in the general population, the researchers found.

What's more, the duration of the interval of decreased colorectal cancer risk persisted for more than 10 years. The Canadian team did not call for a change in the recommended interval, but did say the "findings suggest that screening colonoscopies do not need to be performed at intervals shorter than 10 years."

The Seattle study took a similar look at 1,244 people who underwent colonoscopies, in three age groups: 50 to 54, 75 to 79, and 80 and older.

The incidence of the cancer was much higher in the oldest participants -- 14 percent for the 80-and-up group -- compared to 3.2 percent for the 50-to-54 group. However, the benefit in terms of extended lifespan from colonoscopy was very small for the oldest group -- about 45 days, compared to more than 10 months for those aged 50 to 54.

The results "suggest that the benefit of screening colonoscopy in very elderly patients may be smaller than what is commonly believed," the researchers wrote. They add that the information might "help avoid its use in patients who are unlikely to benefit substantively."

Neither study is definitive, Church said. He agreed that the Canadian finding "doesn't give us any reason to change the currently recommended interval of 10 years. It does leave open the possibility that an even longer interval might be possible."

On the other hand, he said, the Seattle study "raises a legitimate question: What should be the determination of when you no longer screen for colorectal cancer?"

Any answer would involve more factors than simply age and colonoscopy frequency, he said. Other, less uncomfortable screening techniques, such as fecal blood tests and flexible sigmoidoscopy, are known to be effective, Church said.

"The decision needs to be based not strictly on age, but the health of the individual," he said. "Some older people are in good shape and will live for 20 years or more. Others have multiple health problems, their hearts or other organs may be failing. They are likely to die of something other than colorectal cancer, so we definitely need to take into account what a person's health status is before making a screening decision."

Robert Smith, director of screening for the American Cancer Society, said the Manitoba findings support the current recommendation for frequency of screening. "Some people have questioned the 10-year period," Smith said.

As for screening the elderly, "the big issue is identifying those people in whom the benefit outweighs the risk," he said.

"For this age group we have to be extra careful," agreed Dr. Stephen Shibata, director of the gastrointestinal program at the City of Hope Cancer Center, in Duarte, Calif. "We need to know what the patient's health is, what the patient wishes. Patient and physician need to have a discussion before they have a procedure."